Public awareness has increased with respect to the environment, and primary pollutants such as nitrogen oxides and sulfur dioxide are currently regulated in most industries, either under 40 CFR Part 60 or 40 CFR Part 75. It is the responsibility of the federal Environmental Protection Agency and the individual states to enforce these regulations. A great deal of attention in recent years has been spent on addressing the monitoring requirements of these regulations, in order to minimize the discharge of noxious gases into the atmosphere by industrial facilities.
One technique for ensuring correct monitoring of noxious gases has been to implement continuous emissions monitoring systems (CEMS). These systems are utilized to monitor emissions of sulfur dioxide, nitrogen oxides, carbon monoxide, total reduced sulfur, opacity, volatile hydrocarbons, particulate, and heavy metals such as mercury. Typically, a CEMS is installed in the plant at each emissions source. Applicable Federal, state, and local regulations include certain options for continuous monitoring of each of these emissions sources, and regulatory agencies are provided with a monitoring plan for each plant that details how the emission rate is to be measured and reported prior to startup.
A CEM system typically includes either an in situ analyzer installed directly in an exhaust stack, the exhaust pipe of the reciprocating engine, or in an extractive system which extracts a gas sample from the exhaust stack and conveys it to an analyzer at grade level. Continuous emissions monitoring system components such as gas analyzers are quite expensive, difficult to maintain, and difficult to keep properly calibrated. As such, the regulations that deal with a CEM system require the analyzers to be calibrated periodically and subjected to other quality assurance programming to ensure the accuracy and reliability of the compliance data.
In many cases, the regulations allow for certification and operation of alternatives to the hardware-based continuous emissions monitoring system. Such alternatives include software solutions that predict the emissions from available process and ambient parameters. Procedures for certifying these predictive emissions monitoring systems (PEMS) are detailed in the regulations, namely 40 CFR Part 75, Subpart E and 40 CFR Part 60, Appendix B, Performance Specification 16. Generally, a PEM system models the source of emissions that generates the emissions and predicts the quantity of emissions that are produced given the operating state of the process.
Regulations allow a maximum downtime of ten percent for calibration. If a unit remains in operation greater than ten percent of the time with the CEMS down, the emissions level is considered by the regulators to be at maximum potential level. This results in out-of-compliance operation and over-reporting of emissions. Facilities must maintain and operate their gas analyzers to avoid penalties requiring an ongoing operational expense and, occasionally, emergency services are required. A reliable software-based PEMS that can be certified under 40 CFR Part 75, Subpart E would represent an extremely cost-effective option of the compliance monitoring needs of industrial facilities.
There have been PEM systems built in the past to predict various combustion and emission parameters from continuous industrial processes and to calculate process or combustion efficiency for compliance reporting and process optimization purposes. Typically, the PEM system is “trained” by monitoring multiple inputs such as pressures, temperatures, flow rates, etc., and one or more output parameters such as NOx, CO, O2, etc. After training, in normal operation, the PEM system monitors only the multiple inputs and calculates estimated output parameter values that closely match the actual pollutant levels. Methodologies used in the past include nonlinear statistical, neural network, eigenvalue, stochastic, and other methods of processing the input parameters from available field devices and to predict process emission rates and combustion or process efficiency. For the most part, these PEM systems are complicated, relatively costly, and difficult to implement. These systems also typically require retraining with the support of specialized staff from the system provider to adjust the proprietary model to the real-world conditions encountered in the field.